The Effect of the Aminic Substituent on the Thermal

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e Pb. 2+ dithiocarbamates obtained from cyclic amines, is described under nitrogen and air atmospheres in order to investigate the effect of a cyclic ring on the ...
J. Braz. Chem. Soc., Vol. 10, No. 1, 65-75, 1999. Printed in Brazil.

© 1999 Soc. Bras. Química 0103 – 5053 $6.00 + 0.00 Article

The Effect of the Aminic Substituent on the Thermal Decomposition of Cyclic Dithiocarbamates Éder T.G. Cavalheiroa*, Massao Ionashirob, Glimaldo Marinoc, Susete T. Breviglieric, and Gilberto O. Chiericec a

Depto. Química, UFSCar, São Carlos, São Paulo, CP 676,

13565-905 São Carlos - SP,Brazil; [email protected]; b

Instituto de Química, UNESP, Araraquara, São Paulo, CP 355, 14800-900 Araraquara – SP, Brazil

c

Inst. de Química de S. Carlos, USP, São Carlos, São Paulo, CP 780, 13560-970 São Carlos – SP, Brazil Received: September 29, 1998

Estudos termogravimétricos e calorimétricos diferenciais para ditiocarbamatos de NH4+, Na+, Zn , Cd2+ e Pb2+, derivados de aminas cíclicas contendo nitrogênio como heteroátomos, foram realizados em atmosferas de ar e nitrogênio, para avaliar a influência da tensão angular dos anéis na decomposição térmica destes compostos, em relação à formação de tiocianatos metálicos como via de decomposição. Os intemediários formados foram caracterizados por difração de raios-X, tendo sido encontrados oxissulfatos de Zn2+, Cd2+ e Pb2+, sob atmosfera de ar, o que sugere a decomposição térmica nestas condições como via sintética para estes compostos. Os produtos de decomposição final obtidos foram sulfetos metálicos sob nitrogênio e óxidos dos metais de transição e sulfato de sódio sob ar. Entalpias de fusão são também descritas, com base nos resultados de DSC. 2+

Thermogravimetric and Differential Scanning Calorimetric investigation of the thermal behaviour of NH4+, Na+, Zn2+, Cd2+ e Pb2+ dithiocarbamates obtained from cyclic amines, is described under nitrogen and air atmospheres in order to investigate the effect of a cyclic ring on the mechanism of decomposition. Intermediates were identified by X-ray Diffraction analysis. Zn2+, Cd2+ e Pb2+ oxysulphates were detected under air atmosphere suggesting the thermal decomposition under these conditions as an alternating synthetic route to prepare those compounds. The final decomposition products were the metallic sulphides under N2 atmosphere while transition metal oxides and sodium sulphate were obtained under air. Melting enthalpies are also reported from DSC data.

Keywords: dithiocarbamates, thermogravimetry, differential scanning calorimetry, decomposition mechanism

Introduction Dithiocarbamates (DTC), are products of a reaction of a primary or secondary amines with carbon disulphide, and has been described in several applications such as medicine, industry, agriculture and chemistry, that are commented in several reviews1-9. In those reviews is possible to note that the knowledge of DTC thermal behaviour and information about their

stability and decomposition residues, are important data and can show synthetic routes for unusual compounds. In three reviews about the thermal behaviour of DTC compounds1,2,4, most of references are related with the aliphatic derivatives in which a decomposition mechanism involving metallic thiocyanate as the main intermediate. However, few studies has been presented in relation to the cyclic DTC, such as pyrrolidinedithiocarbamate (Pyr) and

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piperidinedithiocarbamate (Pip), in which the formation of the thiocyanate must be related with cleavage of C-N bond in the ring. Pyr complexes are widely used in solvent extraction and other analytical procedures, because of their resistance to acidic media10. In applications such as in atomic absorption spectrophotometry, the thermal decomposition data can help in understanding more about atomization mechanisms of those complexes11. The Pip complexes of Zn and Cd are largely applied as curing agents in rubber processing12 and in photographic films13,14 were the thermal behaviour is an important feature. Recently Fernandez-Alba et al.15, reported studies on thermal decomposition of NH4Pyr and other DTC, with agricultural applications. Vibrational Spectroscopy was used to propose an structure to the complexes. Diaz et al.16 studied decomposition fragments in mass spectroscopy for Bi, Cd, Co, Cr, Cu, Fe, Ni, Pb, Sb, Zn and ammonium in Pyr compounds. Gomicek et al.17, presented X-ray diffraction studies for the characterisation of decomposition products of several Pyr complexes with divalent cations. The decomposition products were obtained at 80, 150, 250, 300, 400 and 600 °C, under oxygen and argon atmospheres. The compounds were obtained in an oven without use of the TG to follow up the thermal decomposition pathways. The same authors18, investigated the thermal behaviour of Cd, Co, Cu, Fe, Ni, Pb and V - Pyr complexes in graphite tubes for atomic absorption. Sceney et al.19, studied the thermal decomposition of seven copper(II) dithiocarbamates by thermogravimetry, under air and nitrogen atmospheres. Complete TG and DTA studies are reported to copper(II)-diethyldithiocarbamate complex under air, vacuum and nitrogen. It is also important to note that few DSC results, about thermal DTC behaviour has been previously reported. The most thermal differential data are from DTA results. In the present work the thermal behaviour of ammonium, sodium, zinc, cadmium and lead complexes with Pyr and Pip under nitrogen and air atmospheres was investigated with the purpose of evaluating the influence of the different angular tension of the 5 and 6 members aminic rings on the decomposition and identify the intermediates formed.

Experimental Syntesis of the ligands and complexes

J. Braz. Chem. Soc.

NaPyr and NH4Pyr) and then dried in a vacuum oven at 50 °C, for 8 h. The metallic complexes were obtained by direct reaction of the DTC sodium salt and a soluble salt of the desired cation. The resulting precipitates were filtered, washed with acetone and dried as above. Equipment Characterazation of the compounds was performed using Vibrational Spectroscopy (KBr pellets) in a Nicolet 5SXC spectrophotometer; NMR-1H in a Brücker AC-200 spectrometer and Flame Atomic Absorption Spectroscopy in an Intralab AA12/1475 (Gemini) spectrophotometer and Elemental Analysis using a Fisons EA 1108 CNHS-O instrument. TG curves were recorded in a DuPont 9900 thermoanalyser coupled with a TGA 951 Thermogravimetric Module under a gas flow of 200 mL min-1 (N2 or air), in a platinum crucible, at 10 °C min-1 heating rate and using samples of about 7 mg of each compound at atmospheric pressure. For the ammonium salts (Pyr and Pip), the TG experiments were carried out only in nitrogen. The DSC curves were recorded in a DuPont 9900 thermoanalyser coupled with a DSC 910 Calorimetric Module under a gas flow of 200 mL min-1 (N2 or air), in aluminium hermetic pans, at 10 °C min-1 heating rate and using samples of about 5 mg of each compound at atmospheric pressure. X-ray diffraction patterns of intermediates and final products, were obtained in an HZ-Karl Zeiss or a D5.000 Siemens diffractometers. The melting points were also determined in a dry furnace Electrothermal digital melting point apparatus. Fusion enthalpies were determined by DSC after calibration of the equipment with indium metal. The area of the melting peak for this metal was stored in the system and compared with the area of sample melting peaks in a standard procedure. Temperatures are also measured in relation to the In fusion, calibration.

Results and Discussion The structural formula of the ligands are: N

C

C S-

S-

Pyr

Pip

The proposition of the earlier works for thiocyanate1 as the decomposition intermediate implies that the decomposition of the DTC complexes occurs with rupture of C-N-C bonds. For a divalent cation-Pyr complex we can propose: S

Ammonium and sodium salts of the ligands, were prepared by direct reaction between the piperidine or pyrrolidine and carbon disulphide in basic media and recrystallised from acetone (NH4Pip) or ethanol-water 1:1 (v/v) (NaPip,

S

S N

N

S

C

C S

Me

S

N

heat

M(SCN)2 + 2S + 2

(1)

The differences in the angular tension between the 5 and 6 membered rings of pyrrolidine and piperidine respec-

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Thermal Decomposition of Cyclic Dithiocarbamates

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Figure 1. TG (solid line) and DTG (dashed line) curves for the Pyrrolidinedithiocarbamates under N2 atmosphere: a) NH4+; b) Na+; c) Zn2+; d) Cd2+; e) Pb2+; and under air: f) Na+; g) Zn2+; h) Cd2+ and i) Pb2+.

tively suggests that Pyr derivatives, a more tense 5 member ring, are supposed to decompose preferentially by the thiocyanate. However the results obtained here suggest the decomposition in a direct way (except to CdPyr2), without the thiocyanates as the main decomposition pathway, in agreement with some observations of Fernandez-Alba et al.15 The spectrometric characterisation results are shown in Table 1 and the elemental analysis data for all the compounds are shown is in Table 2. The TG-DTG curves for Pyrrolidinedithicarbamates are shown in Fig. 1 under nitrogen and air atmospheres. The DSC curves under both atmospheres are in Fig. 2. Mass losses, temperature ranges and residues for the decomposition of all Pyr compounds are described in Ta-

ble 3. The melting points and enthalpies are shown in Table 4. NH4Pyr The TG/DTG curves of ammonium salt showed a single decomposition step between 70 and 175 °C (Fig 1.a) without residues on the crucible. The DSC curves of ammonium salt showed endothermic peaks at 139, 164, 191 and 199 °C under nitrogen atmosphere (Fig. 2.a). Under air (Fig. 2.f) the peaks are at 141, 163, 186 and 195 °C. In order to attribute the peaks a sample was heated in a glass tube (20 x 2 cm) immerse in a glycerine bath. Around 140 °C the gas produced by heating the sample was bubbled into a phenolphtalein solution turning it to red and showed a characteristic smell of ammonia26. Near 150 °C melting

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of sample was observed producing a brown viscous liquid, which IR spectra suggests that it is the HPyr (Table 1). The gas at 165 °C when bubbled in a Sb3+ solution produced an orange solid and showed a characteristic smell of H2S. The colour is characteristic for Sb2S3 26.

J. Braz. Chem. Soc.

According to these observations it was shown that the NH4Pyr decomposes by the following steps: NH4Pyr → NH3 + Hpyr

(2)

HPyr(s) → HPyr(l)

(3)

Table 1. Molecular absorption spectrometry (UV), infrared spectrometry (IR) and nuclear magnetic resonance 1H (NMR) data obtained for the compounds.

IR bands / cm-1

UV

Compound

ν(C=S)

λMAX/nm (log ) HPyr 264(4.0)-280(4.0)a

NH4Pyr

264(4.0)-280(4.0)

NaPyr

a

ZnPyr2

ν(N-CS2)

ν(C-N)

996

1160

1455

996

1159

(1005c)

998

1003 994

Cd Pyr2 Pb Pyr2 a

NH4Pip

256(4.1)-278(4.0)

NaPip

256(4.1)-278(4.0)a

NMR

1160

993

1117

(1003c)

(1420c)

1.8; 3.5

(1485d)

2.0; 3.7

(1475e)

f

(1460e)

1509

2.0; 3.8

1400

1.6; 1.8; 3.2; 3.6

1421

1482

1160 1162

1.9; 3.4; 3.8

1392 (1129c)

1161

1000

ν / ppm

1471

(1110c)

(1465b)

1470

1.4; 2.0; 4.0

1113

1484

1.5; 1.7; 4.0

975

1112

1482

1.5; 1.7; 4.1

973

1002

1464

1.5; 1.7; 3.9

1005

1110

ZnPip2

979

Cd Pip2 Pb Pip2

a. in agreement with Ref. 20, 21; b. Ref. 22; c. Ref. 23; d. Ref. 24; e. Ref. 25; f. insoluble in deutered water.

Table 2. Analytical data of the compounds (%).

Metal

C

H

N

calc.

-

36.55

7.38

17.05

found

-

36.59

7.61

17.11

calc.

11.20

29.25

5.90

6.82

Compound NH4Pyr NaPyr.2H2O ZnPyr2 Cd Pyr2 Pb Pyr2 NH4Pip NaPip.2H2O ZnPip2 Cd Pip2 Pb Pip2

found

10.90

29.31

5.83

6.87

calc.

18.26

33.55

4.51

7.89

found

18.79

33.57

4.62

7.81

calc.

27.76

29.66

3.99

6.92

found

27.20

29.69

3.90

6.88

calc.

41.46

24.03

3.23

5.61

found

40.50

24.15

3.30

5.66

calc.

-

40.41

7.93

15.71

found

-

39.98

7.84

15.79

calc.

10.48

32.85

6.45

6.39

found

10.79

32.92

6.51

6.25

calc.

16.94

37.34

5.23

7.26

found

17.46

37.41

5.19

7.21

calc.

25.96

33.29

4.66

6.47

found

25.80

33.21

4.60

6.52

calc.

39.26

27.31

3.83

5.31

found

40.69

27.40

3.74

5.28

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Table 3. Mass loses and temperature range corresponding to the decomposition of the pyr compounds.

Temp. Interval / °C

Thermal Process

Mass Loss or residue/ % TG

Calc.

N2 Atmosphere NaPyr.2H2O → NaPyr + 2H2O NaPyr → Na2S3.3

61-131 340-394

ZnPyr2 → ZnS (+ Zn)

243-389

ZnS → Zn

389-811

CdPyr2 → CdS (+ C)

280-421

CdS(S) → CdS(g)

566-961

PbPyr2 → PbS

17.5

36.5

b

37.0

22.0

b

25.9

18.5

b

18.3

42.5

b

35.7

0

272-390

PbS(s) → PbS(g)

17.6a

b

49.5

673-1030

0

3.52 b

48.0

b

9.02

Air Atmosphere NaPyr.2H2O → NaPyr + 2H2O NaPyr → Na2SO4 ZnPyr2 → ZnS

71.7-130

17.5a

17.5

217-324

35.2

b

34.6

26.7

b

27.2

30.4

b

-

21.7

b

22.7

33.9

b

35.7

37.5

b

37.0

31.4

b

31.7

51.2

b

-

59.0

b

60.7

250-479

ZnS → ZnSO3 + ZnS

540-594

ZnSO3 + ZnS → ZnO

711-788

CdPyr2 → CdS

277-470

CdS → Cd3OSO4

564-670

Cd3OSO4 → CdO

779-942

PbPyr2 → PbS + PbSO4

272-367

PbS → PbSO4

367-711

- not calculated, mixture; a: relative to a mass loss; b: relative to the residue.

Table 4. Melting points and energies, determined by dsc and capillary tubes.

Compound

Atmosphere

DSC Temperature / °C

Capillary / °C -1

∆fusHm / J mol

NH4Pyr

N2/air

163.8

*

150

ZnPyr2

N2

293.8

25.3

294

air

293.6

25.3

PbPyr2

N2

286.0

37.5

air

285.1

34.2

NaPip

N2

301.7

19.7

air

**

**

N2

228.4

36.9

air

228.2

35.4

CdPip2

N2

258.2

30.3

air

258.3

30.1

PbPip2

N2

244.6

37.5

air

258.1

35.9

ZnPip2

* base line not clear for integration procedure; ** masked by the decomposition process.

286 296 228 259 244

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Figure 2. DSC curves for the Pyrrolidinedithiocarbamates under N2 atmosphere: a) NH4+; b) Na+; c) Zn2+; d) Cd2+; e) Pb2+; and under air: f) NH4+ g) Na+; h) Zn2+; i) Cd2+ and j) Pb2+.

HPyr(l) → H2S + other volatiles

(4)

As the DSC experiments were carried out in an hermetic crucible the temperatures are higher than the observed in the open tube. These observations are not described in the earlier reports. When the DSC experiment is carried out in an open crucible, a single peak at 155 °C is observed, under both atmospheres. In the TG experiments these features are not observed probably because the crucible is open. NaPyr Sodium salt showed the decomposition in two steps, relative to the loss of 2H2O, with sodium polissulphide and a carbonaceous residue under nitrogen atmosphere (340394 °C) according to Figure 1.b and sodium sulphate air (Fig. 1.f), and it was the most stable compound in the series here studied. The presence of the polisuphide is in agree-

ment with the observed in earlier works27,28, and were characterised by X-ray diffraction. The DSC curves for sodium salt evidenced peaks attributed to a loss of water and decomposition under N2 (Fig. 2.b). The experiment carried out in air (Fig. 2.g) showed the loss of water followed by exothermic processes of decomposition and oxidation to Na2SO4, respectively. ZnPyr2 TG/DTG curves of zinc compound showed decomposition in a single step resulting mainly in ZnS and a small amount of Zn ° in the residue under nitrogen (Fig. 1.c). This mixture is slowly reduced up to 811 °C producing Zn°. Under air (Fig. 1.g) three steps were observed. The first was the decomposition and formation of ZnS, followed by

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Thermal Decomposition of Cyclic Dithiocarbamates

production of ZnSO3 and ZnS and pure ZnO. All these data were confirmed by X-ray diffraction data. The DSC curves under N2 atmosphere (Fig. 2.c), showed a endothermic peak at 269 °C (∆H = 8.32 kJ mol-1), attributed to a crystalline phase change followed by a melting process, coincident with the observation in a capillary tube. The endothermic peak at 331 °C, was attributed to decomposition of the compound. Under air atmosphere (Fig. 2.h) endothermic peaks at 268 °C (∆H = 8.40 kJ mol-1), and at 294 °C, were attributed to crystalline phase change and melting respectively, in agreement with the data under nitrogen atmosphere. The decomposition of the complex is represented by endothermic peaks at 325 and exothermic at 346 °C. ZnPyr2, is the unique compound that showed crystalline phase transition in the present series. CdPyr2 The TG curves in N2 atmosphere (Fig. 1.d) showed the decomposition with generation of CdS and a amorphous material according the X-ray diffraction data. The black amorphous material was insoluble in HCl and was suspect to carbonaceous residue. The DTG curve suggests that the first step is split in two consecutive reactions. This could be related with the formation of thiocyanide considering that the Cd2+ is soft acid with affinity to soft base represented by that anion, according to Pearson’s concept29 and the weakening of the C-N ring bonds by angular tension. Above 566 °C volatilisation of the CdS is observed, represented by a yellow substance impregnating the exhausting furnace tube, with black residue of 3.6% at the crucible, probably carbon. Under air atmosphere (Fig. 1.h) the residues showed the presence of CdS at 400 °C and pure Cd3OSO4 at 700 °C. The final residue is CdO. All these results were confirmed by X-ray diffraction data. The DSC curves do not evidenced the occurrence of melting in agreement with capillary tube observation. Under N2 atmosphere (Fig. 2.d), is possible to note the decomposition, related with the endothermic peaks at 320 and 345 °C. Under air atmosphere (Fig. 2.i) decomposition is represented by the peaks at 309, 336 °C and 346 °C in agreement with TG data. The exothermic processes at 446 °C, was attributed to carbon burning. PbPyr2 Under nitrogen atmosphere (Fig. 1.e) the PbPyr2 TG curves showed two steps. The first one resulting in a black residue of PbS, confirmed by X-ray diffraction measurements and by smell of H2S in presence of hydrochloric acid. The second step was related to volatilization of the PbS; resulting in a small amount of intermetallic Pb-Pt at crucible, at 1030 °C, according the X-ray diffraction data.

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Under air atmosphere (Fig. 1.i) the decomposition occurred with formation of a mixture of PbS and PbSO4, followed by oxidation to PbSO4 and a small amount lead oxysulphate, Pb3O2SO4, according the X-ray diffraction results. The DSC curve under N2 atmosphere (Fig. 2.e), showed a sharp endothermic peak, attributed to the melting, followed by an endothermic decomposition peak at 321 °C. The curve in air (Fig. 2.j) also suggests a melting process. In the sequence occurred the decomposition of compound with peaks at 320 °C and at 334 °C, followed by the oxidation process represented by the exothermic peak at 453 °C. The TG-DTG curves for Piperidinedithicarbamates are shown in Fig. 3 under nitrogen and air atmospheres. The DSC curves under both atmospheres are in Fig. 4. Mass losses, temperature ranges and residues for the decomposition of all Pyr compounds are described in Table 5. The melting points and enthalpies are shown in Table 4. NH4Pip The TG/DTG curves of this salt showed a single mass loss (85-180 °C), without residues in the crucible (Fig. 3.a). The DSC curves showed a sharp endothermic peak, at 172 °C and 175 °C under N2 (Fig. 2.a) and air (Fig. 2.f) atmospheres respectively, without evidence of melting. Experiment in an open glass tube showed that the compound sublimates without decomposition since the IR spectra of the sublimate product is identical to the original salt. NaPip This salt showed a very similar behaviour with the NaPyr. The decomposition occurred in two steps under N2 (Fig. 3.b). The first relative to the loss of 2H2O, in agreement with Kudela et al. report23. The second step is related to decomposition and formation of sodium polisulphide as residue under nitrogen27,28. Under air (Fig. 3.f) the TG curve showed the water loss was and formation of sodium sulphate in the range. All these residues were confirmed by X-ray diffraction, being the most stable piperidine derivative in the series here studied. The DSC curves for sodium salt showed three endothermic peaks under N2 atmosphere (Fig. 4.b). The first at 142 °C related to the water loss, the second a sharp peak at 302 °C, attributed to a melting process. The last one at 338 °C, was attributed to the decomposition. Under air (Fig. 4.g) showed the loss of water (endothermic peaks at 148 and 165 °C), followed by complex exothermic processes of decomposition represented by the peaks at 291 and 320 °C and oxidation to Na2SO4 at 417 °C. The melting is probably masked by the strong exothermic decomposition of the compound under oxidative atmosphere.

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Table 5. Mass loses and temperature range corresponding to the decomposition of the pip compounds.

Thermal Process

Temp. Interval / °C

Mass Loss or residue/ % TG

Calc.

N2 Atmosphere NaPip.2H2O → NaPip + 2H2O NaPip → Na2S2 ZnPip2 → ZnS (+ Zn) ZnS → Zn CdPip2 → CdS + C CdS(S) → CdS(g) PbPip2 → PbS PbS(s) → PbS(g)

16.3a

16.4

25.2

b

25.1

24.6

b

25.2

16.8

b

17.0

37.2

b

33.4

3.90

b

0

46.0

b

45.3

702-1030

3.47

b

0

66-130

16.1a

16.4

32.0

b

32.4

25.0

b

-

23.1

b

-

20.3

b

21.0

37.5

b

33.4

35.7

b

34.5

30.1

b

29.7

49.5

b

45.3

52.0

b

50.2

57.9

b

57.5

64-126 297-362 228-400 541-951 240-423 660-954 232-394

Air Atmosphere NaPip.2H2O → NaPip + 2H2O NaPip → Na2SO4 ZnPip2 → ZnS + ZnSO3 ZnS + ZnSO3→ ZnO + ZnSO4 ZnSO4 → ZnO CdPip2 → CdS + C CdS → Cd3OSO4 Cd3OSO4 → CdO PbPip2 → PbS (+ Pb2OSO4) PbS → Pb2OSO4 Pb2OSO4 → PbSO4

233-324 226-379 379-444 690-748 241-360 482-739 799-916 253-389 389-508 549-730

- not calculated, mixture; a: relative to a mass loss; b: relative to the residue.

ZnPip2

CdPip2

TG/DTG curves of zinc compound showed decomposition in a single step under nitrogen atmosphere (Fig. 3.c) resulting in a white residue confirmed by HCl test and X-ray diffraction as the. This is slowly reduced to Zn°. Under air atmosphere (Fig. 3.g) there was three steps of decomposition, resulting in a mixture of ZnS and ZnSO3 at 379 °C; ZnO and ZnSO4 at 444 °C and ZnO at 748 °C. All these compounds were identified by their X-ray diffraction patterns. The DSC curves under N2 atmosphere (Fig. 4.c), showed a endothermic peaks at 228 and 332 °C, attributed to melting and decomposition of the compound respectively. Under air atmosphere (Fig. 4.h) endothermic peak at 228 °C is observed related to the melting, in agreement with nitrogen experiments. The decomposition of the complex is represented by the peaks at 329 and at 347 °C. The melting point is in disagreement with that of Oktavec et al.30, who describes that it occurs at 288-289 °C.

The TG/DTG curves showed that the decomposition occurred in two steps under N2 atmosphere (Fig. 3.d) generating CdS which is present as a yellow residue, confirmed by X-ray diffraction and HCl test. The difference between the TG and calculated residues is due to the presence of carbonaceous residue, represented by amorphous phase in X-ray diffraction. Finally volatilization of CdS is observed, with a black residue of 3.9% exactly the difference at 423 °C, attributed to the carbonaceous material. Under air atmosphere (Fig. 3.h) the main decomposition step occurred producing CdS and carbonaceous material followed by oxidation lead to the Cd3OSO4 and finally CdO as the final product of decomposition. All these products were characterised by X-ray diffraction patterns. The DSC curves evidenced the occurrence of melting represented by the peaks at 258 °C under N2 (Fig. 4.d) and at 258 °C under air atmosphere (Fig. 4i). Decomposition were represented by the endothermic peak at 323 under N2.

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Figure 3. TG (solid line) and DTG (dashed line) curves for the Piperidinedithiocarbamates under N2 atmosphere : a) NH4+; b) Na+; c) Zn2+; d) Cd2+; e) Pb2+; and under air: f) Na+; g) Zn2+; h) Cd2+ and i) Pb2+.

Under air the peaks at 321 and 335 °C were attributed to the decomposition, and the exothermic peak at 448 °C was related with the production of Cd3OSO4. PbPip2 The TG/DTG curves under nitrogen (Fig. 3.e) for the lead compound showed a decomposition, resulting in a black residue of PbS confirmed by X-ray diffraction and test with HCl. Above 700 °C, volatilization of the PbS occurred, similarly to the PbPyr2. Under air atmosphere (Fig. 3.i) a decomposition occurred, with formation of a mixture of PbS and traces of lead oxysulphate Pb2OSO4. Then the mixture is converted to Pb2OSO4, and finally PbSO4. The residues were identified by X-ray diffraction.

The DSC curve under N2 atmosphere (Fig. 4.e), showed a sharp endothermic peak at 244 °C attributed to melting, followed by a decomposition peak at 323 °C. The curve under air atmosphere (Fig. 4.j) showed the melting peak. At the sequence the decomposition of the compound is observed with peaks at 316 °C, 333 °C, and 416 °C.

Final Comments It was expected that the angular tension in the fivemembered pyrrolidine ring, should lead to a weakening of the C-N ring bonds favouring its rupture generating the metallic thiocyanate but this is not true. According to the results obtained here, both cyclic DTC, decomposed preferentially in a direct way, without the formation of a thiocyanate intermediate, in a different

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J. Braz. Chem. Soc.

Figure 4. DSC curves for the Piperidinedithiocarbamates under N2 atmosphere: a) NH4+; b) Na+; c) Zn2+; d) Cd2+; e) Pb2+; and under air: f) NH4+ g) Na+; h) Zn2+; i) Cd2+ and j) Pb2+.

behaviour described to non cyclic derivatives1. The CdPyr suggested the unique exception. The most remarkable difference in the decomposition as a function of the aminic group was observed for the ammonium salts. Some oxysulphates were obtained as products, suggesting that the thermal decomposition of the dithiocarbamate complexes may be an useful synthetic route to obtain such compounds.

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